Output product solubility constants

[ammonge]

Hello,

I am trying to output the product solubility constants as a function of temperature for dissolution of calcite in pure water in a closed system. How can I print the logKsp values either to a file or in a plot? I seem unable to find the option to either plot it or print it with SELECTED_OUTPUT. This is the little code I am using for this purpose:

Code: [Select]

TITLE Calcite solubility product constant as Ksp=f(T)
SOLUTION 1 Pure water             
EQUILIBRIUM_PHASES 1
        Calcite         0.0     1.0
REACTION_TEMPERATURE 1
        0.0 225.0 in 226 steps
SELECTED_OUTPUT
        -file   CalciteKsp_temp.sel
        -temperature
USER_GRAPH 1 Calcite Ksp=f(T)
        -chart_title "Calcite Ksp=f(T)"
        -axis_scale x_axis 0 225 5 0
        -axis_scale y_axis auto 0.05 0.1
        -axis_titles "Temperature, in degrees celsius" "Saturation index"
        -initial_solutions false
  -start
  10 graph_x TC
  20 graph_y SI("Calcite")
  -end
END

If I wanted to do a similar calculation, but after equilibrating the solution with a gas with a certain partial pressure of CO2(g), do you have any recommendations about it?

Many thanks in advance.

[dlparkhurst]

Regardless of the reactions or gases you define, the SI will be 0.0 provided there is enough calcite to dissolve to equilibrium. Also, the log K will be the same at each temperature unless you are also changing pressure. Phreeqc.dat and pitzer.dat have the molar volume data to calculate log K as a function of pressure in addition to temperature.

Code: [Select]

TITLE Calcite solubility product constant as Ksp=f(T)
SOLUTION 1 Pure water             
EQUILIBRIUM_PHASES 1
        Calcite         0.0     1.0
REACTION_TEMPERATURE 1
        0.0 225.0 in 226 steps
USER_GRAPH 1 Calcite Ksp=f(T)
    -headings               tc SI logK(Calcite)
    -axis_titles            "Temperature, in degrees celsius" "Saturation index" "log K"
    -chart_title            "Calcite Ksp=f(T)"
    -axis_scale x_axis      0 225 5 5
    -axis_scale y_axis      auto 0.05 0.1 0.1
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 graph_x TC
20 graph_y SI("Calcite")
30 GRAPH_SY LK_PHASE("Calcite")
  -end
    -active                 true
END

[ammonge]

I am a bit confused and it may well be that my mistake is conceptual. I thought the temperature dependency of equilibrium constants could be modelled either using polynomials or the reaction enthalpies. Isn't that so?

[dlparkhurst]

Sorry, I meant that the temperature-dependence curve of log K will be the same, regardless of which reactions you define.

[ammonge]

It?s clear now. Many thanks.